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2. 2
Objectives
1. Explain the meaning of fluid power.
2. List the various applications of fluid power.
3. List the advantages and disadvantages of fluid power.
4. Explain the industrial applications of fluid power.
5. Differentiate between mechanical ,electrical, pneumatic and
hydraulics systems.
6. Differentiate between hydraulics system and pneumatic
7. Energy losses in hydraulic systems.
8. ISO symbols
Unit-1

3. 3
Methods for transmitting power
Mechanical transmission Electrical transmission Fluid power
eg:shafts, gears, chains, belts eg: wires, transformers eg: liquids or gas
Fluid Power:
Def: the technology that deals with the generation,
control and transmission of forces and movement
of mechanical element or system with the use of
pressurized fluids
- Both liquids and gases are considered as fluids

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5. 5
Advantages of a Fluid Power System:
1. Fluid power systems are simple, easy to operate
and can be controlled accurately
2. Multiplication and variation of forces
3. Multifunction control
4. Low-speed torque
5. Economical
6. Low weight to power ratio
7. Fluid power systems can be used where safety
is of vital importance

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Fluid power system includes –
1. a hydraulic system (hydra in Greek
meaning water) and
2. a pneumatic system (pneuma in Greek
meaning air).

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Fluid power applications can be classified into two major
segments:
Stationary hydraulics:
fixed in one position
valves are mainly solenoid
operated
Applications:
1. Machine tools and transfer
lines.
2. Lifting and conveying
devices.
3. Metal-forming presses.
4. Plastic machinery such as
injection-molding machines.
5. Rolling machines.
6. Lifts.
7. Food processing machinery.
8. Automatic handling
equipment and robots.
Mobile hydraulics:
move on wheels or tracks
valves are frequently manually
operated
Applications:
1. Automobiles, tractors ,
aéroplanes, missile, boats ,
etc.
2. Construction machinery.
3. Tippers, excavators and
elevating platforms.
4. Lifting and conveying
devices.
5. Agricultural machinery.

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S. No. Hydraulics System Pneumatics System
1
It employs a pressurized liquid
as a fluid
It employs a compressed gas, usually
air, as a fluid
2
An oil hydraulic system operates at
pressures up to 700 bar
A pneumatic system usually operates
at 5–10 bar
3 Generally designed as closed system Usually designed as open system
4
The system slows down when leakage
occurs
Leakage does not affect the system
much
5
Valve operations are difficult Valve operations are easy
6
Heavier in weight Lighter in weight
7
Pumps are used to provide
pressurized liquids
Compressors are used to provide
compressed gases
8 The system is unsafe to fire hazards The system is free from fire hazards
9
Automatic lubrication is provided
Special arrangements for lubrication
are needed

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Types of hydraulic systems-
1. Hydrostatic Systems:
 uses fluid pressure to transmit power
 The pump used is a positive displacement pump
 An example of pure hydrostatics is the transfer of force in
hydraulics.
2. Hydrodynamic Systems:
 use fluid motion to transmit power
 The pump used is a non-positive displacement pump.
 An example of pure hydrodynamics is the conversion of flow
energy in turbines in hydroelectric power plants.

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11. A typical hydraulic system
11
1 – pump
2 – oil tank
3 – flow control valve
4 – pressure relief valve
5 – hydraulic cylinder
6 – directional control valve
7 – throttle valve

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PROPERTIES OF FLUID:
 Density: defined as mass per unit volume
• density changes with pressure and decreases with temperature
Eg:
At 20°C, for example, the density of water changes from 998 kg/m3 at
1 atm to 1003 kg/m3 at 100 atm, a change of just 0.5 percent.
At 1 atm, for example, the density of water changes from 998 kg/m3
at 20°C to 975 kg/m3 at 75°C, a change of 2.3 percent,

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Specific Weight: defined as weight per unit volume
Specific Volume : volume occupied by a unit mass of
fluid

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Specific Gravity : defined as the density of the given fluid divided by
the density of water

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Surface tension:
The cohesive forces between liquid molecules are responsible for
the phenomenon known as surface tension.

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 Viscosity:
The viscosity of a fluid is a measure of its resistance to
shear or angular deformation.

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Viscosity index (VI) :
It is a relative measure of the change in the viscosity
of an oil with respect to a change in temperature.
An oil having a low VI is one that exhibits a large
change in viscosity with a small change in temperature.
A high VI oil does not change appreciably with a
change in temperature.

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The various properties required for an ideal
hydraulic fluid are as follows:
1. Ideal viscosity.
2. Good lubrication capability.
3. Demulsibility.
4. Good chemical and environmental stability.
5. Incompressibility.
6. Fire resistance.
7. Low flammability.
8. Foam resistance.
9. Good heat dissipation.
10. Low density.
11. System compatibility.

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Lubrication Capability:
Hydraulic fluids must have good lubricity to prevent friction and
wear between the closely fitted working parts such as vanes of pumps,
valve spools, piston rings and bearings.

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Demulsibility
The ability of a hydraulic fluid to separate rapidly from moisture and
successfully resist emulsification is known as “demulsibility.”
If an oil emulsifies with water, the emulsion promotes the
destruction of lubricating and sealant properties.
Highly refined oils are basically water resistant by nature.
Good Chemical and Environmental Stability (Oxidation and
Corrosion Resistance) :
Most fluids are vulnerable to oxidation, as they come in contact with oxygen in air.
Mineral oils or petroleum-based oils (widely used in hydraulic systems) contain
carbon and hydrogen molecules, which easily react with oxygen.
The oxidation products are highly soluble in oil and being acidic in nature they can
easily corrode metallic parts

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Neutralization Numbers :
is a measure of the acidity or alkalinity of hydraulic oil.
This is referred to as the pH value of the oil. High acidity
causes the oxidation rate in oil to increase rapidly.
Incompressibility:
hydraulic fluids as incompressible, in practice, they are relatively
compressible.
 Most mineral oils undergo reduction in the volume of
about 0.7% for every 100 bar rise in pressure.
the compressibility of a fluid is greatly influenced by temperature
and pressure.

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Types of Hydraulic Fluids :
1. Petroleum-based fluid
2. Emulsions
3. Water glycol
4. Synthetic fluids
5. Vegetable oils
6. Biodegradable hydraulic fluids

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1. Petroleum-based fluid:
Mineral oils are the petroleum-based oils
Advantage:
1. they are easily available and are economical
2. they offer the best lubrication ability ,
3. least corrosion problems and are compatible with most
seal materials
Disadvantage:
Flammability:
They pose fire hazards, mainly from the leakages, in
high-temperature environments such as steel industries, etc.

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2.Emulsions:
a mixture of two fluids that do not chemically react with others
 Emulsions of petroleum-based oil and water are commonly used.
An emulsifier is normally added to the emulsion, which keeps liquid as small
droplets and remains suspended in the other liquid.
Two types of emulsions are in use:
a).Oil-in-water emulsions:
water as the main phase, while small droplets of oil are dispersed in it
the oil dilution is limited, about 5%; hence, it exhibits the
characteristics of water.
Limitations: poor viscosity, leading to leakage problems, loss in
volumetric efficiency and poor lubrication properties.
These problems can be overcome to a greater extent by using certain
additives. Such emulsions are used in high-displacement, low-speed
pumps (such as in mining applications).

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b) Water-in-oil emulsions/inverse emulsions:
 basically oil based in which small droplets of water are dispersed
throughout the oil phase.
The commonly used emulsion has a dilution of 60% oil and 40% water
popular fire-resistant hydraulic fluids
 exhibit more of an oil-like characteristic; hence, they have good
viscosity and lubrication properties.
These emulsions are good for operations at 25°C, as at a higher
temperature, water evaporates and leads to the loss of fire-resistant
properties.

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3. Water glycol:
nonflammable fluid commonly used in aircraft hydraulic systems.
has a low lubrication ability as compared to mineral oils and
is not suitable for high-temperature applications.
It has water and glycol in the ratio of 1:1.
Because of its aqueous nature and presence of air, it is prone to
oxidation and related problems.
It needs to be added with oxidation inhibitors.
Enough care is essential in using this fluid as it is toxic and corrosive
toward certain metals such as zinc, magnesium and aluminum.

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4. Synthetic fluids:
based on phosphate ester, is another popular fire-resistant fluid.
It is suitable for high-temperature applications, since it exhibits good
viscosity and lubrication characteristics.
It is not suitable for low-temperature applications.
It is not compatible with common sealing materials such as nitrile.
5. Vegetable oils:
biodegradable and are environmental safe.
They have good lubrication properties, moderate viscosity and are
less expensive good fire resistance characteristics with certain additives,
tendency to easily oxidize and absorb moisture.
The acidity, sludge formation and corrosion problems are more severe
in vegetable oils than in mineral oils.
Hence, vegetable oils need good inhibitors to minimize oxidation
problems

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6. Biodegradable hydraulic fluids / bio-based hydraulic fluids :
Bio-based hydraulic fluids use sunflower, soybean, etc.,
as the base oil and hence cause less pollution in the case of
oil leaks or hydraulic hose failures.
These fluids carry similar properties as that of a mineral
oil–based anti-wear hydraulic fluid,

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Factors Influencing the Selection of a Fluid:
1. Operating pressure of the system.
2. Operating temperature of the system and its variation.
3. Material of the system and its compatibility with oil used.
4. Speed of operation.
5. Availability of replacement fluid.
6. Cost of transmission lines.
7. Contamination possibilities.
8. Environmental condition (fire proneness, extreme atmosphere
like in mining, etc.).
9. Lubricity.
10. Safety to operator.
11. Expected service life.

30. Hydraulic fluids - tasks
30
They have the following primary tasks:
Power transmission (pressure and motion
transmission)
Signal transmission for control
Secondary tasks:
Lubrication of rotating and translating components to
avoid friction and wear
Heat transport, away from the location of heat
generation, usually into the reservoir
Transport of particles to the filter
Protection of surfaces from chemical attack, especially
corrosion

31. Hydraulic fluids - requirements
31
 Functional
Good lubrication characteristics
Viscosity should not depend strongly on
temperature and pressure
Good heat conductivity
Low heat expansion coefficient
Large elasticity modulus
 Economic
Low price
Slow aging and thermal and chemical stability 
long life cycle

32. Hydraulic fluids - requirements (contd.)
32
 Safety
High flash point or in certain cases not
inflammable at all
Chemically neutral (not aggressive at all
against all materials it touches)
Low air dissolving capability, not inclined to
foam formation
 Environmental friendliness
No environmental harm
No toxic effect

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Ideal and real fluid

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1. Laminar flow/streamline
In streamline flow, the fluid appears to move by sliding of
laminations of infinitesimal thickness relative to adjacent layers;
that is, the particles move in definite and observable paths or
streamlines.

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2.Turbulent flow:
It is characterized by a fluid flowing in random way. The movement of
particles fluctuates up and down in a direction perpendicular as well as
parallel to the mean flow direction.

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Reynolds Number
If Re is less than 2000, the flow is laminar.
If Re is greater than 4000, the flow is turbulent.
Reynolds number between 2000 and 4000 covers a
critical zone between laminar and turbulent flow.

38. Governing laws
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e) Continuity
b) Pascals’s law
g) Bernoulli
equation
f) Flow resistance
a) Hydrostatic pressure c) Transmission of power
d) Transmission of
pressure

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Distribution of fluid power:
Steel Pipes:
extensively used in fluid power systems, although they are
rapidly being supplemented by steel or plastic tubing.
disadvantages of steel pipes are their weight and the large
number of fitting requirement for connection .
advantage is its mechanical strength and particularly its
ability to withstand abuse.

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Screwed Connections :
Steel piping in fluid power systems is most often joined by
threaded connections.
Steel Tubing :
widely used material for hydraulic system conductors.
it can be easily formed to fit irregular paths so that fewer
fittings are required.
lessened chance of leakage since every connection is a
potential leak point.
It is also relatively small and light, thus making it easy to use.

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Compression Joints :
comprise a loose ring having a cone-shaped nose that must face the
open end of a tube, a mating tapered barrel and a retaining nut.
The end of the tube must always be cut square and deburred before
assembly.
 When the tube is pushed fully in the fitting and the retaining nut is
tightened, the compressive action forces the nose of the ring into the
surface of the metal tube,
creating a permanent and very strong
interference fit that is capable of withstanding
pressure in excess of 350 bar.

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Plastic Conductors:
available in polyethylene, polypropylene, polyvinyl chloride and nylon
compatible with most hydraulic fluids, however, and could safely be used in low-
pressure applications.
Flexible Hoses :
A hose is manufactured from natural and synthetic rubbers and several plastics.
This material is supported by fabric or by wire cloth, and wire braid may be used
between plies or as an outside casing for high-pressure applications

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Quick Disconnect Couplings :
This type of coupling in conjunction with flexible hoses connects
movable components together hydraulically.
Typical applications are mobile trailers and agriculture machinery
towed behind tractors.
usually comprise a plug and socket arrangement that provides a
leak-proof joint when two parts are connected together, and that can
be released easily without the use of tools
 Each half of the coupling contains a spring-loaded ball or poppet
that automatically closes on disconnection, so that two completely
leak-free joints are obtained.
 Leaking during the process of disconnecting or connecting
coupling is negligible

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Types of Quick Couplings:
There are three basic types of quick couplings;
1. single shut-off,
2. double shut-off, and
3. straight-through

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Single shut-off couplings/One-Way shut-off or Pneumatic couplings:
 installed with the valved half on the pressure side of the circuit to provide
automatic shut-off flow when the coupling is disconnected.
low working pressure capabilities ranging from 100 to 300 PSI.
The are commonly made from brass or steel.
Applications -lubrication, paint spray, and carpet cleaning equipment.

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Double Shut-off Couplings /Two-way shut-off / Hydraulic
Couplings:

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3. Straight-through coupling:

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BENDS:

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ENERGY LOSSES IN HYDRAULIC SYSTEMS:
Darcy–Weisbach Equation :
Head losses in a long pipe in which the velocity distribution
has become fully established or uniform along its length can
be found by Darcy’s equation as
Where, f is the Darcy friction factor,
L is the length of pipe (m),
D is the inside diameter of the pipe (m),
v is the average velocity (m/s) and
g is the acceleration of gravity (m/s2).

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Frictional Losses in Laminar Flow:
Darcy’s equation can be used to find head losses in pipes
experiencing laminar flow by noting that for laminar flow, the friction
factor equals the constant 64 divided by the Reynolds number:
Substituting this into Darcy’s equation gives the Hagen–Poiseuille
equation:

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Equivalent Length :
length of pipe that for the same flow rate would produce the same
head loss as a valve or fitting.
where , Le is the equivalent length of a valve or
fitting.

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Effect of Pipe Roughness
The relative roughness of pipe is defined as the ratio of inside surface
roughness to the diameter:
Here, (ε) is surface roughness

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Frictional Losses in Valves and Fittings
Where, K is called the loss coefficient of valve or fittings

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Seals:
Fun:
Used to prevent both internal and external leakage of fluid
Prevent dirt, Dust enters into system
Types:
Static : no relative movement occurs between mating parts
Dynamic: movement occurs
Classification by shape:
1. O-ring
2. Quad-ring
3. T-ring
4. V-cup ring
5. Hat ring
6. U-cup ring

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1. O-ring:
widely used seal for hydraulic
systems.
It is a molded synthetic rubber
seal that has a round cross-
section in its free state
used for the most static and dynamic conditions.
It gives effective sealing through a wide range of pressures,
temperatures and
movements with the added advantages of sealing pressure
in both directions and providing low running friction on
moving parts.

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Figure : Relative position of O-ring packings in different grooves
at increasing pressure.

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Quad-ring

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T-ring:
dynamic seal that is extensively used to seal cylinder-pistons,
piston rods and other reciprocating parts
made of synthetic rubber molded in the shape of the cross-
section T and reinforced by backup rings on either side
The sealing edge is rounded and seals very much like an O-ring.

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V-ring seal and U-ring seal:
This are compression-type seals used in virtually in all types of
reciprocating motion applications like, piston rods and piston seals in
pneumatic and hydraulic cylinder, press rank, jacks and seals on
plungers and piston in reciprocating pumps.
Figure (a)V-ring seal and (b) U-ring seal

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Piston cup packings:
designed specifically for pistons in reciprocating pumps and pneumatic
and hydraulic cylinders.
best service life for this type of application, require a minimum recess
space and minimum recess machining, and can be installed easily and
quickly.

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Sources of Hydraulic System Contamination
New Fluid – most new fluid is not acceptable for use in hydraulic systems and
must be filtered first
Built-In – contamination introduced into the system during the manufacture,
assembly and testing of components
Ingressed – external ingression of atmospheric contamination; air condenses
and water is released into the reservoir
Induced – particles introduced during normal maintenance or system
operation
In-Operation – wear generation contamination caused by the pump,
actuators, cylinder or the hydraulic motor
Rubber and Elastomers – degradation of rubber compounds and elastomers
products
High Water Based Fluids – supports biological growth
Replacement of Failed Components – failure to thoroughly clean conductor
lines after replacing a failed pump